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Old 05-05-2019, 02:26 PM
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Frame / motor mounts

70 Monte Carlo. I switched from a small block which was the original engine, to a big block. I did also change the trans from a th-350 to a 4L80E and used a crossmembers.com crossmember which should keep the mount and pinion angle right where it should be. From what I can tell, using a magnetic angle gauge the angle is exactly zero which is perfect! But my engine looks like it’s tilted back some. I thought I remember reading that engine angle should be about 5 degrees, I’m at about 8.

So my question is, I can certainly place a couple shims between the crossmember and the trans to lift it up some and the pinion angle would probably be ok and the engine wouldn’t look so tilted, but of course I’d rather not change the zero pinion angle, so...... are big block frame mounts where the motor mounts connect different than small block frame mounts? I thought I read someone saying that but I’ll be damned if I can find a listing for them. I’m thinking if bb frame mounts are shorter cuz the engine is taller it will lower my engine and solve the engine angle of 8 degrees down some. Thanks for the advice/opinions. Much appreciated!

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Old 05-05-2019, 03:07 PM
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According to this, BBC and 350/400 SBC cars share mounts on the '68-72 A/G-body chassis, only the earlier stuff had an actual different frame stand.
Good explanation and pics:

https://www.chevellestuff.net/tech/engine_mounts.htm

If it was a 307 car(don't know if they did that in the Monte?) the engine mounted piece is different, narrower than the 350/400 and BBC piece, and the frame stand that goes with the low performance mount matches that narrow width AND is also taller.

Easiest way to look up the correct frame mount is probable 1970 SS 396/454 Chevelle, it will be the same parts used on your Monte.

yes, shimming the crossmember is an accepted fix...I think it is even mentioned on Crossmembers.com.
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Old 05-05-2019, 03:17 PM
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Hi Eric. I don’t find a listing for different motor or frame mounts for chevelle either. I always check Chevelle listings as well when looking for parts for my Monte.
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Old 05-05-2019, 03:35 PM
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I’d guess we’d need to assume your pinion angle on the rearend is up 8 degrees.
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Old 05-05-2019, 03:40 PM
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Rwenuts....when I placed the angle gauge on the trans it’s at zero. When I place it at the rear end where the ujoint sits, it’s zero. When I place it on the engine lift plate which is still bolted to the intake, it’s at 8 degrees.
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Old 05-05-2019, 03:48 PM
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All that tells me is your gauge is fubar.

Where are you measuring on the tranny and the rearend?
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Old 05-05-2019, 03:55 PM
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On the trans across the tail shaft and the trans pan. At the read end across the area where the ujoint sits in the pinion yoke.
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Old 05-05-2019, 04:13 PM
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That explains it.
You need to measure on the machined surfaces.
Info here.
https://www.drivelinesnw.com/part-tr...ngle-problems/
I usually measure on the horizontal surface of the yoke.
Also your carb flange should be level if you have a carbed motor.

More good info.
https://www.pirate4x4.com/tech/billa...ft/index2.html
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Old 05-05-2019, 04:23 PM
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Wouldn’t the horizontal surface of the yoke angle be the same as the angle across the opening in the tail shaft? There can’t be any difference in the 2 because the yoke is slid in there and right against the tail shaft.
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Old 05-05-2019, 04:48 PM
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Yes!
Horizontal or vertical should be the same.
You can measure either way. Shows that in the first link.
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Old 05-05-2019, 04:50 PM
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Then how is the measurement off?
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Old 05-05-2019, 05:13 PM
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It’s off(not zero) because your motor is sitting tilted back at 8 degrees.

What intake do you have?
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Old 05-05-2019, 05:19 PM
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Sounds correct to me, on the rear and trans angles you want those equal and opposite or with an allowance of tipping the pinion down compared to the trans 2-3 relative to the transmission to allow for axle wrap and pinion climb when power is applied...you will often find a different angle on the intake carb face, especially with aftermarket intakes.
Some are compensated to sit the carb level and some are not.

Trans pan bottom of case is often not square with the shaft centerline, don't use those surfaces as the pan or rail is also often compensated for engine installed angle.

Unusual to see pinion and trans at both zero if measuring relative to ground...typically it will be something like trans down 5 relative to ground and pinion up 3, with allowance then there for 2 of axle wind-up.
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Old 05-05-2019, 05:22 PM
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Holley Dominator. Like I was saying, I could shim the tail of the trans up. The crossmember came with shims. This should bring the angle closer to level on the engine.
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Old 05-05-2019, 05:24 PM
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Eric... yes I was too wondering if the pan was a safe place to check the angle. That’s why I checked the angle across the opening of the tail shaft where the yoke slips in.
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